Many semiconductor processes use or generate solid, condensable or subliming compounds. For example, low-pressure chemical vapour deposition silicon nitride (LPCVD nitride) processes tend use silanes (such as disilane or trisilane) and ammonia to produce a uniform layer of silicon nitride to insulate the substrate. These processes tend to produce a very thick film of silicon nitride, and consequently require very long deposition cycles, typically 3 to 8 hours. As a result, a lot of powder is generated as a by-product of this process. Such by-products include complex ammonium-chloro-silicate salts, for example, ammonium hexasilicate, which sublimes at 120° C. at atmospheric pressure.
As these materials enter the inlet of a wet scrubber they cool, and can agglomerate and react with water vapour back streaming from the wet scrubber. For example, ammonium hexasilicate can form a glass like deposit lining the inlet of the wet scrubber. If deposit build-up is allowed to continue uninterrupted, it can completely block the inlet, incurring down time and loss of production. For instance, most process tool manufacturers (OEMs) mechanically clean the process chamber off-line, which allows the solid deposits to build up uninterrupted.